<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 200px;
            height: 200px;
            display: flex;
            justify-content: center;
            align-items: center;
            background-color: antiquewhite;
        }
        .btn{
            width: 100px;
            height: 100px;
            background-color: aqua;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="btn"></div>
    </div>
</body>
<script>
    let btn = document.getElementsByClassName('btn')
    
</script>
</html>